The control and/or elimination of undesirable impurities and by-products from various manufacturing operations has gained considerable importance in view of the potential pollution such impurities and by-products may generate. One conventional approach for eliminating or at least reducing these pollutants is by oxidizing them via incineration. Incineration occurs when contaminated air containing sufficient oxygen is heated to a temperature high enough and for a sufficient length of time to convert the undesired compounds into harmless gases such as carbon dioxide and water vapor.
In view of the high cost of the fuel necessary to generate the required heat for incineration, it is advantageous to recover as much of the heat as possible. To that end, U.S. Pat. No. 3,870,474 (the disclosure of which is herein incorporated by reference) discloses a thermal regenerative oxidizer comprising three regenerators, two of which are in operation at any given time while the third receives a small purge of purified air to force out any untreated or contaminated air therefrom and discharges it into a combustion chamber where the contaminants are oxidized. Upon completion of a first cycle, the flow of contaminated air is reversed through the regenerator from which the purified air was previously discharged, in order to preheat the contaminated air during passage through the regenerator prior to its introduction into the combustion chamber. In this way, heat recovery is achieved.
U.S. Pat. No. 4,302,426 discloses a thermal regeneration anti-pollution system which adjusts for excessive temperatures in the high temperature incineration or combustion zone. To that end, the temperature in the combustion zone is sensed, and when a predetermined high temperature is reached therein, the gases that normally would be passed through the heat exchange bed are instead bypassed around the bed, then combined with other gases that have already been cooled as a result of their normal passage through a heat exchange bed, and are exhausted to atmosphere.
However, basing the bypass operation on the temperature sensed in the combustion or high temperature zone is somewhat inefficient, and can result in undesirable heat spikes. Further problems with such regenerative thermal apparatus are lack of uniform heat distribution in the combustion chamber, and the expense of using a burner or electric heater to maintain combustion chamber temperature. Specifically, typically one or more burners are placed in the combustion chamber to control the combustion chamber temperature. However, areas of the combustion chamber that are not in proximity to the burners tend to be cooler than those areas in proximity to the burners. Areas of lower temperature can result in incomplete combustion of the contaminated process air. Providing additional burners to overcome the problem is not an economical solution. Moreover, continual operation of the burners is the main source of NO.sub.x generated.
U.S. Pat. No. 4,267,152 discloses the injection of natural gas into the contaminated air before it enters the combustion chamber to promote more uniform heat distribution. Combustion chamber temperature is sensed, and once that temperature attains a predetermined level sufficient for auto-ignition of the natural gas, the natural gas injection into the contaminated air is carried out. If the combustion chamber temperature exceeds a predetermined level, the natural gas injection is ceased. This operation is coordinated with burner operation, the latter also being based upon sensed combustion chamber temperature.
However, even with the thermocouple sensing the combustion chamber temperature, low temperatures in areas of the combustion chamber not in proximity to the thermocouple can go undetected, thus leading to incomplete combustion. Similarly, it is also possible for higher than necessary temperatures that are not in the proximity of the thermocouple to go undetected. The presence of higher than necessary temperatures can result in excessive fuel consumption and pressure. Moreover, basing the gas injection on a predetermined temperature limit within a margin of error to ensure complete combustion is inherently inefficient.
It is therefore an object of the present invention to improve the efficiency of such regenerative thermal oxidizers.
It is a further object of the present invention to promote uniform temperature distribution in such regenerative thermal oxidizers.
It is a still further object of the present invention to regulate the thermal heat recovery efficiency of the heat exchange zones of a regenerative thermal oxidizer to insure optimum fuel consumption in the oxidation process.
It is yet a further object of the present invention to minimize and even eliminate the operation of the combustion zone burner once steady state operation has been achieved.
It is a further object of the present invention to trim the thermal heat recovery by way of bypassing the downstream heat exchange zone to adapt the thermal efficiency to the needs of the industrial effluent.
It is still another object of the present invention to regulate the contaminant concentration of the industrial effluent through the dilution of the industrial effluent prior to entering the regenerative thermal oxidizer.